"Among Twitter users, the median number of lifetime tweets per user is one."

Those that do tweet tend to do so during traditional US work hours, with messaging dipping on weekends, according to HubSpot.

Common phrases used in tweets indicate the San Francisco startup's service is most popular in cities where the dominant language is English, said HubSpot, which noted that Twitter continued to grow at "an astounding" rate.

Twitter has been adding millions of users a month and its website received 32.1 million unique visitors in April, according to tracking firm comScore.

Twitter, which allows members to exchange 140-character-or less messages, can be used on computers, mobile telephones, and on television sets using a newly-announced online link to Microsoft Xbox 360 videogame consoles.